Silicon block, method for producing the same, crucible of transparent or opaque fused silica suited for performing the method, and method for the production thereof

ABSTRACT

A method for producing a solar crucible includes providing a crucible base body of transparent or opaque fused silica having an inner wall, providing a dispersion containing amorphous SiO 2  particles, applying a SiO 2 -containing slip layer to at least a part of the inner wall by using the dispersion, drying the slip layer to form a SiO 2 -containing grain layer and thermally densifying the SiO 2 -containing grain layer to form a diffusion barrier layer. The dispersion contains a dispersion liquid and amorphous SiO 2  particles that form a coarse fraction and a fine fraction with SiO 2  nanoparticles. The weight percentage of the SiO 2  nanoparticles based on the solids content of the dispersion is in the range between 2 and 15% by weight. The SiO 2 -containing grain layer is thermally densified into the diffusion barrier layer through the heating up of the silicon in the crystal growing process.

TECHNICAL FIELD

The present invention refers to a silicon block with a monocrystallineor polycrystalline structure.

Moreover, the present invention refers to a method for producing such asilicon block in a crystal growing process for silicon by providing asolar crucible with a crucible base body of transparent or opaque fusedsilica comprising an inner wall, of which at least a part is covered bya SiO₂-containing grain layer, the silicon being filled into the solarcrucible, the silicon being heated so as to form a silicon melt, and thesilicon melt being cooled down with crystallization and formation of thesilicon block.

Furthermore, the present invention refers to a solar crucible with arectangular shape for use in a crystal growing process, the cruciblecomprising a crucible base body of transparent or opaque fused silicacomprising an inner wall, and a SiO₂-containing layer which covers theinner wall at least in part.

Moreover, the invention is concerned with a method for producing a solarcrucible with a rectangular shape for use in a crystal growing process,comprising providing a crucible base body of transparent or opaque fusedsilica which comprises an inner wall, providing a dispersion containingamorphous SiO₂ particles, applying a SiO₂-containing slip layer with alayer thickness of at least 0.1 mm to at least a part of the inner wallby using the dispersion, drying the slip layer so as to form aSiO₂-containing grain layer and thermally densifying the SiO₂-containinggrain layer so as to form a diffusion barrier layer.

PRIOR ART

Blocks of monocrystalline or polycrystalline silicon for solarapplications are fused according to the Bridgman or Vertical GradientFreeze (VGF) method in solar crucibles of transparent or opaque fusedsilica.

These solar crucibles are often produced with the help of ceramic slipmethods and have a polygonal, in the simplest case, a rectangular,crucible opening. For reasons of costs the crucibles are predominantlyproduced from naturally occurring quartz raw material. Therefore, thepurity of the crucible material is basically less than that ofsynthetically produced transparent fused silica. In the crystal growingprocess at least partial heating up to temperatures above 1,410° C. iscarried out into the crucible wall. The impurities existing in thecrucible wall cause crystallization with formation of cristobalite.During cooling, a structural transformation known as “cristobalite jump”leads to the destruction of the crucible, which can thus only be usedonce.

During cooling the silicon block shrinks more than the crucible. In caseof adhesions to the crucible wall, stresses and cracks may occur in thesilicon block. This is countered with separation layers of Si₃N₄, SiC,or the like.

Moreover, metallic impurities are also introduced from the crucible wallinto the silicon in the crystal growing process. They are introduced byway of volume diffusion, surface diffusion or by evaporation of volatilemetal compounds from the fused silica body, namely into the liquidsilicon phase and also into the crystallized solid phase. In the pastyears the content of impurities has been reduced constantly and the mainimpurity iron, which is also representative of other metallic impuritiessuch as e.g. Cr or Cu, has been lowered from about 100 wt. ppm to 10 wt.ppm. Iron impurities are however particularly critical as they impairthe electronic properties of the semiconductor material and diffuseparticularly easily.

Therefore, on the solidified silicon block (also called “ingot”) onefinds a region of increased Fe total concentration in the peripheralzone. This concentration is typically in the range of 10¹⁴ to 10¹⁶at/cm³. Under unfavorable circumstances the peripheral zone representsloss of material that significantly contributes to the manufacturingcosts for solar cells. (Note: the literature frequently givesconcentration data only for the iron interstitially dissolved in thesilicon lattice, but the above data refer to the total concentration ofiron).

The contamination can be deduced from the deteriorated electricalproperties in the peripheral region. In the measurement by means of theminority-carrier charge carrier lifetime (LTDL) the bad region is shownin red and is therefore also called “red zone”. Peripheral regions withreduced electrical cell efficiency are detected in the processed cellwith the help of the photoluminescence measurement (PL) and are visibleas dark regions.

To reduce the input of impurities from the crucible wall, the inner wallof the crucible is provided with diffusion barriers. A great number ofmethods are known for this.

It is e.g. suggested in DE 10 2011 082 628 A1 that the bottom and thesidewalls of the crucible should be covered with a diffusion barrier inthe form of inserted plates of transparent fused silica of a high puritywith thicknesses of e.g. 1 mm. The plates of transparent fused silicacan additionally be provided on the inside with a Si₃N₄ layer tosimplify the removal of the silicon block.

Open gaps that must be sealed remain between the individual plates andin the Peripheral region. There is a high risk of breakage for theplates due to irregularities in the crucible and point loads aftersilicon filling. Undefined thermal transitions arise from differentsupport gap widths. Moreover, plates of transparent fused silica of ahigh purity are very expensive. High material costs and the mountingefforts make the manufacturing process expensive, especially for theprovision of large-volume crucibles with widths of more than 0.5 m andwall heights of 0.25 m and more.

EP 0 949 358 A1 suggests a method for producing a crucible, in which awax core in the form of the inner wall of the crucible is provided and aslip layer is applied thereto by dipping, the layer containingfine-grained SiO₂ particles with particle sizes of 40 μm as well ascolloidal SiO₂. Fine fused silica sand of high purity with particlesizes of around 150 μm is sprinkled by addition onto the slip layer.Dipping and sprinkling are repeated until a layer thickness of 3 mm isreached. Coarse fused silica sand with particle sizes between 500 μm and1500 μm of lower purity is sprinkled after repeated dipping into theslip. This process will, also be repeated until a layer thickness of 8mm is reached. The layer composite produced thereby, which consists ofan inner layer and an outer layer, is baked at a temperature of 800° C.for 2 hours after fusion of the wax core. In the crystal growing processit is only the inner layer that gets into contact with the silicon,adheres to the silicon block upon cooling and delaminates from the outerlayer. The formation of mechanical stresses in the silicon block isthereby avoided.

A similar method is also known from WO 2005/106084 A1. It describes acoating of the bottom and the inner walls of a solar crucible base bodywith an aqueous slurry that contains SiO₂ particles of different sizes.The SiO₂ layer has a thickness of up to 500 μm and serves as anintermediate layer. The surface layer as such consists of Si₃N₄.Adhesion of the SiO₂ intermediate layer to the base body is deliberatelykept low, so that it delaminates during cooling and the silicon blockremains undamaged and is kept free from stresses as much as possible.The low adhesion of the intermediate layer is generated by conferring acertain porosity to said layer.

U.S. Pat. No. 5,053,359 A describes a sintered crucible of amorphousSiO₂ powder of high purity which has added to it as a dopant acrystallization promotor in the form of aluminum oxide. The aluminumoxide addition effects crystallization of the crucible wall during themelting process. Additions of crystallization promotors are alsodescribed for fused silica crucibles that are vitreously molten in thearc process, for instance in WO 2007/063996 A1. The crystal formationprimarily serves to enhance the temperature stability. It is also notedthat contaminations arising from the crucible wall can thereby bereduced.

DE 10 2009 049 032 B3 describes a method for producing a component ofcoated quartz glass—for example, a crucible—by spraying a SiO₂containing slurry on a surface. The slurry contains both splintery andspherical amorphous SiO₂ particles and it contains SiO₂ nanoparticleswith a weight fraction between 0.2 and 10% by wt. The particles consistof at least 99.99% by wt. of SiO₂. The cristobalite content is not morethan 0.1% by wt. The slurry composition is optimized with respect toviscous flow for coating vertical and inclined surfaces by spraying.

OBJECT OF THE INVENTION

The last-mentioned patent refers to a fused silica crucible for thecrystal pulling process according to the Czochralski method. Thecrucibles used therein are rotation-symmetrical and are fused in an arcprocess. After production of the base body a thin inner layer ofhigh-purity SiO₂ is often applied thermally to the inside in anadditional step so as to improve the surface quality. However, onaccount of their polygonal inner cross-section, the arc melting processis not suited for solar crucibles.

Solar crucibles are typically solidified by sintering in a sinteringfurnace. However, during sintering there is the risk that the sinteringprocesses for densifying the crucible wall and possible crystallizationprocesses overlap one another, thereby impeding the densification of thecrucible wall, with a residual porosity remaining in the structure. Inthe case of a porous structure, however, impurities can particularlyeasily pass into the melting material.

These problems basically arise also during sintering of the diffusionbarrier layers produced by using particulate start material according tothe above-explained prior art, especially if these are thermallydensified only during the crystal growing process; thus, it must beensured not only that a sufficiently dense layer is created, but alsothat during the heating-up phase and the early melting phase the amountof impurities passing from the crucible wall into the melt is as smallas possible.

It is not easy to satisfy these conditions, so that cells from theperipheral region of silicon blocks frequently still exhibit a dark celledge, i.e. a zone of reduced efficiency when said edge is made visibleby photoluminescence measurement.

It is therefore the object of the present invention to provide a siliconblock of high purity and to indicate a method which permits aninexpensive manufacture of such a silicon block with a low loss ofmaterial.

Furthermore, it is the object of the present invention to provide asolar crucible that reliably satisfies this demand.

Moreover, it is the object of the present invention to provide aninexpensive method for producing such a solar crucible.

SUMMARY OF THE INVENTION

As for the method for producing the crucible by using a dispersioncontaining amorphous SiO₂ particles, this object starting from a methodof the aforementioned type is achieved according to the invention in

-   (a) that the dispersion contains a dispersion liquid and amorphous    SiO₂ particles that form a coarse fraction with particle sizes in    the range between 1 μm and 50 μm and a fine fraction with SiO₂    nanoparticles with particle sizes of less than 100 nm, wherein the    weight percentage of the SiO₂ nanoparticles based on the solids    content of the dispersion is in the range between 2 and 15% by wt.,-   (b) and that the SiO₂-containing grain layer is thermally densified    into the diffusion barrier layer through heating-up of the silicon    in the crystal growing process.

For the above-explained reasons, particularly because of thecrystallization tendency of the crucible base body and because of thehigh temperatures needed for densification, the preparation of a densediffusion barrier layer from SiO₂-containing grain layer before theintended use of the crucible encounters difficulties. In the methodaccording to the invention the SiO₂-containing grain layer is thermallydensified into the diffusion barrier layer only in the crystal growingprocess.

Hence, a particular challenge of the method according to the inventionconsists in providing a SiO₂-containing grain layer which is free ofimpurities, if possible, and which upon heating up in the crystalgrowing process can be densified rapidly into a diffusion barrier layerthat is as dense as possible. During heating up of the silicon in thecrystal growing process the crucible wall reaches temperatures rangingbetween 1,370° C. and 1,450° C. The maximum temperature on the cruciblewall is normally just above the melting temperature of silicon, but itmay also be lower in the bottom area of the crucible.

This procedure offers the advantage of avoiding thermal stress of thecrucible base body which might crystallize and form cracks in thisprocess. To be more specific, cracks caused by the cooling process mayarise subsequent to possible thermal coating processes. Therefore, theformation of the dense diffusion barrier layer during the siliconmelting process has the advantage that it is created without theformation of cracks. The sintering of the green layer already starts atrelatively low temperatures; a still open-pore sinter layer is firstobtained. A significant densification which leads to a closed-poreformation of the sinter layer begins above the transformation rangewhich, depending on the type of transparent fused silica of the grainlayer, is in the temperature range of 1,000° C. to about 1,200° C. Thedensified sinter layer corresponds to the diffusion barrier layer; itcan relax thermally introduced stresses and thereby counteract theformation of cracks. A precondition is that the diffusion barrier layeris vitreous-amorphous and does not crystallize. When the meltingtemperature for silicon is reached (1,410° C.), the vitreous-amorphousdensified sinter layer has a viscosity (in the order of 10¹⁰ dPas), atwhich a significant plastic deformability is given.

To reduce the “red zone” in the silicon block significantly, small ironcontents of less than 2 wt. ppm, preferably of less than 0.5 wt. ppm,are needed. It has been found that this requires not only a densediffusion barrier layer, but the density of the diffusion barrier layermust also be provided as rapidly as possible during the heating up ofthe silicon charge, i.e. if possible, before the maximum temperature isreached in the crystal growing process.

Although sintering aids or binders might contribute to a rapid sinteringof the grain layer, they would simultaneously introduce impurities intothe slip layer and thereby promote the crystallization tendency of thelayer and are therefore not suited. Nevertheless, to achieve a rapid andreliable densification of the diffusion barrier layer, the inventionprovides a particularly sinter-active grain layer. Its high sinteringactivity is achieved by way of a multimodal grain size distribution inthe case of which amorphous SiO₂ particles with particle sizes in therange between 1 μm and 50 μm (coarse fraction) and SiO₂ nanoparticles(fine fraction) with a weight percentage between 2 and 15% by wt. areprovided.

The sintering capacity of the grain layer depends particularly on thecomposition of the slip layer in the near-surface region. Thedistribution and the amount of SiO₂ nanoparticles are here decisive. Ahigh amount yields an increased sinter activity that permits a thermaldensification at a comparatively low temperature or during a shortsintering period into a glass of enhanced density and reduced porosity.Ideally, only relatively fine SiO₂ particles are found in thenear-surface region of the slip layer.

To come close to such a situation, the slip layer is given sufficientopportunity to segregate e.g. during the drying process. Within the sliplayer produced, the segregation effects a division into a lower portionadjoining the inner wall of the crucible, in which the coarse fractionof the SiO₂ particles is predominantly found, and into an outer portionadjoining the free surface of the layer, in which the fine fraction isenriched. The fine fraction of the SiO₂ particles is formed bynanoparticles. Nanoparticles typically consist of a group of a fewthousand SiO₂ molecules and normally have a specific BET surface area inthe range of 50-400 m²/g. In contrast to the above-explained methodaccording to EP 0 949 358 A2, in which several sublayers are produced, asingle layer application is enough in the method according to theinvention; the particles of different particle sizes are here separatedby segregation and other physical effects; these shall be explained inmore detail hereinafter.

Within the slip layer, an inhomogeneous particle size distribution willdevelop soon after application to the inner wall of the crucible,wherein the transition between lower and upper region in the green layershows a perceivable shape under the microscope (the dried slip layer ishere called green layer). Decisive parameters for a successfulsegregation are:

-   -   The initial liquid content of the slip—the content should be        sufficiently high, preferably at least 20% by wt., to ensure an        adequate mobility of the SiO₂ nanoparticles within the slip        layer.    -   The manner how the slip layer is applied. The layer is        preferably built up not successively in several sublayers, as is        e.g. the case when the slip layer is sprayed on (“Aufsprühen”),        but is produced in one operation, preferably by casting or        spraying on (“Aufspritzen”) and with a minimum thickness of 0.1        mm, preferably a minimum thickness of 0.3 mm. This provides, on        the one hand, a sufficiently large reservoir of SiO₂        nanoparticles, and an excessively fast drying, of the layer is        on the other hand prevented, which may counteract adequate        segregation. When the layer is applied by casting or spraying on        (“Aufspritzen”), the slip is applied either as a continuous jet,        i.e. without division into individual drops, or with such a        small division pulse that a reduction of the drop diameter of        the slip below its balance size is avoided; it is at least 1 mm.        Therefore, the liquid content of the slip is not significantly        reduced when the slip layer is applied. The slip layer is here        preferably given its end shape under the action of a tool, such        as a doctor blade, a brush, an injection nozzle or a spatula.        Through the spreading action of the processing tool the layer        surface gets slightly more liquid, which facilitates the        enrichment of SiO₂ nanoparticles also in the case of a        relatively low liquid content. In this connection spray coating        in comparison with casting or spraying on (“Aufspritzen”) shows        another disadvantage because due to the mechanical pulse during        spraying suspension drops with small drop diameters are formed,        in the case of which drying inevitably already starts in the        flight phase to a considerable extent. The liquid content of,        the slip layer formed by spraying therefore differs considerably        from that of the initial slip. The loss of liquid cannot be        easily compensated by raising the initial liquid content because        of the segregation tendency that is then increasing.    -   An amount of SiO₂ nanoparticles that is high enough to effect a        significant enrichment in the outer region of the slip layer—in        the slip if is in the range from 2 to 15% by wt. and preferably        less than 10% by wt. In contrast to the spray layer which is        distinguished by a substantially homogeneous distribution of the        particle sizes over the layer thickness, the slip layer is        preferably inhomogeneous as it shows an inhomogeneous irregular        course of the particle size distribution over the layer        thickness, with an enrichment of SiO₂ nanoparticles in the        near-surface region. In ceramic process engineering, these        near-surface regions are also called “casting skin” and are        often considered to be an indication of an undesired in        homogeneity of the slip layer and are normally removed. Due to        this enrichment the green layer can however be dense-sintered        more easily and rapidly than without it. This means that dense        sintering requires a lower sintering temperature and/or a        shortened sintering duration than in the case of a slip layer        with a homogeneous particle size distribution. At very great        amounts of SiO₂ nanoparticles one can observe a tendency to the        formation of cracks during drying or sintering.    -   The way how the dispersion liquid is removed—this is preferably        done at a slow pace and in a targeted manner in the direction of        the free surface, so that the escaping liquid can entrain SiO₂        nanoparticles upwards into the outer region. Preferably, the        slip layer is dried in that the dispersion liquid is removed at        a rate and in a direction in such a manner that under the action        of the withdrawing dispersion liquid the fine fraction is        enriched in the outer region of the slip layer, while forming a        “casting skin”, as shall be defined hereinafter in more detail.

A particularly homogeneous, dense and sinter-active near-surface volumeportion with a relatively great amount of SiO₂ nanoparticles is therebyproduced within the slip layer, the amount being higher than the averageamount of the SiO₂ nanoparticles in the slip layer.

The enrichment of the finer SiO₂ particles and particularly of the finefraction on the surface of the slip layer is visually perceivable asskin formation, which is here also called “casting skin”. Visually, theslip layer possibly appears to be covered by a wax layer.

The casting skin is also visible after drying (in the green layer). Thesurface region of the green layer that shows a low porosity is hereregarded as the casting skin. The thickness of said skin is preferablyin the range of 3 to 15 μm, particularly preferably in the range of 5 to10 μm. Furthermore, the casting skin is distinguished in that the finefraction consisting of SiO₂ nanoparticles with particle sizes of lessthan 100 nm accounts for a volume proportion of the casting skin of morethan 70%, preferably a volume proportion of more than 80%.

Normally, the SiO₂ nanoparticles are present in the casting skin not inisolated form, but in the form of aggregates or agglomerates that partlyor completely embed the few SiO₂ particles of the coarse fraction. Sincethe porosity of the casting skin is low, the volume proportioncorresponds approximately to the weight percentage of the SiO₂nanoparticles. Thus, a volume proportion of more than 70% in the castingskin corresponds to an enrichment by more than 10 times in comparisonwith a typical initial slip having an average weight percentage of about7% or less of SiO₂ nanoparticles.

It has been found that such a slip layer can be sintered reproduciblyand under moderate sintering conditions (i.e., comparatively lowsintering temperature and/or short sintering period) into a dense,transparent fused silica layer with a relatively low surface roughness.These moderate sintering conditions are normally fulfilled in theintended use of the crucible during heating of the silicon charge beforethe maximum temperature is reached in the crystal growing process, e.g.the melting temperature of the silicon (about 1,410° C.). A low closedporosity of preferably not more than 10% remains in the region of theformer casting skin.

It has been found that such a thin, but dense surface layer alreadyconstitutes an efficient diffusion barrier particularly for iron.

As is generally known, while SiO₂ nanoparticles show a high sinteringactivity, which explains the comparatively low sintering temperature,they nevertheless effect on the other hand a high drying shrinkage at ahigh concentration, namely in lateral direction (in the layer plane),and can thereby lead to delamination of the layer and to the formationof cracks. The fact that these effects do not occur in the methodaccording to the invention can be ascribed to a good interlockingbetween and remaining green layer through the rather large SiO₂particles.

The final densification of the slip layer (or the green layer,respectively) not before the intended use of the crucible simplifies themanufacturing process and avoids an otherwise needed hot treatmentprocess. Fundamentally, this does not rule out a thermal treatment ofthe green layer at a temperature below the maximum temperature in thecrystal growing process, at which a certain densification can also takeplace, such as e.g. for sintering the crucible wall at a temperaturebelow the cristobalite formation temperature or for burning-in a surfacelayer of Si₃N₄, which typically takes place below 1,200° C. However,repeated heating up of the green layer to high temperatures may lead toundesired interactions with adjoining layer regions, so that thisprocedure is in principle not advantageous.

It has been found that upon use of a crucible produced according to theinvention for producing a silicon block in a Bridgman of VFG process,“red zone” and dark cell edge are considerably reduced or avoided.

The sintering activity of the slip layer or of the green layer,respectively, can be further enhanced if SiO₂ grains of syntheticallyproduced transparent fused silica are used having a hydroxyl groupcontent of at least 50 wt. ppm.

Amorphous synthetic SiO₂ grains are distinguished by high purity andshow a lower viscosity at the same temperature in comparison withamorphous SiO₂ grains that have been produced from naturally occurringquartz raw material. The viscosity range of natural SiO₂ grains that isshifted towards elevated temperatures is partly due to the aluminumoxide typically contained therein. Synthetic SiO₂ grains are largelyfree of aluminum oxide. By contrast, hydroxyl groups (OH groups)additionally reduce the viscosity of transparent fused silica. Thus theviscosity profile shifts further to lower temperatures due to the use ofOH-containing SiO₂ grains. As a consequence, the sintering process andthus also the densification of the layer start at comparatively lowtemperatures.

The SiO₂ content of the amorphous SiO₂ particles is preferably at least99.99% by wt. Thus the solids content of the slip produced by using suchSiO₂ particles consists of at least 99.99% by wt. of SiO₂. Binders orother additives are not provided. The total content of metallicimpurities of the transition elements is preferably less than 5 wt. ppm,particularly preferably less than 2.5 wt. ppm, and the content of ironis less than 2 wt. ppm, preferably less than 1 wt. ppm, and particularlypreferably less than 0.5 wt. ppm. This initial material does not poseany risk of contamination or crystallization. The cristobalite amount inthe dried SiO₂ slip layer (=green layer) should be not more than 1% bywt. because, otherwise, the green layer might crystallize duringsintering, which may impede densification and lead to waste of thecrucible.

The smaller the iron content of the SiO₂ grains, the lower is thecrystallization tendency of the grain layer during sintering and thesmaller is the input into the silicon melt. The total iron content ofthe grains is therefore preferably less than 2 wt. ppm, preferably lessthan 1 wt. ppm, and particularly preferably less than 0.5 wt. ppm.

The dispersion liquid preferably has an aqueous basis. The polar natureof the aqueous phase may have an advantageous effect on the interactionof the SiO₂ particles.

It has turned out to be advantageous when the solids content of thedispersion is less than 80% by wt., preferably in the range between70-80% by wt. and quite particularly preferably between 74 and 78% bywt.

This is a relatively low solids content, as is e.g. typical of sprayslip. The spraying capacity requires low viscosity and thus a low solidscontent. On the other hand, such a small solids content is as such notdesired because it may lead to an increased shrinkage of the slip layerand to the formation of cracks. The low solids content, however, makesit easier to transport SiO₂ nanoparticles into the outer region of theslip layer, so that it is preferred in the method according to theinvention even if the dispersion is not used as a spray slip, butanother application technique is used, such as spreading or applicationwith a doctor blade on the inner wall of the crucible, which as suchwould allow the use of a dispersion having rather high solids contents.

Apart from the composition of the slip layer, the removal of thedispersion liquid is an important parameter for the formation of a densecasting skin. In this connection a measure is preferably taken that hasthe effect that the slip layer dries at a slower pace than without thatmeasure.

A slowed-down drying can be achieved in the simplest case in that dryingis carried out in an environment with moisture that is higher than isnormally the case or at a comparatively low temperature. The temperatureof the substrate is often raised for drying, e.g. to more than 100° C.;this rise in temperature can here be omitted. To ensure an adequatesegregation of the SiO₂ nanoparticles on the surface and for theformation of a casting skin an initial drying period of at least 2minutes, preferably at least 3 minutes, has turned out to beparticularly useful.

In the simplest case the crucible base body is a commercially availablesolar crucible of porous transparent fused silica or porous opaque fusedsilica of average purity. As a rule, the crucible wall to be coated istherefore porous and absorbent. In this case, moistening before theapplication of the dispersion will be beneficial if a decrease in thedrying rate is to be achieved. The preceding moistening also helps inthis case to fill open or closed pores with liquid in advance, so thatthe suction effect thereof is reduced in the subsequent application ofthe slip layer, for the withdrawal of the dispersion liquid should takeplace as completely as possible towards the free surface, so that SiO₂nanoparticles can be entrained by the liquid in that direction. In thecase of a suction effect due to pores, SiO₂ nanoparticles would betransported away in the opposite direction. That is why the conceivablealternative, namely a raising of the liquid proportion of the dispersionfor compensating for the suction effect, is quite ineffective and couldmoreover lead—because of the very great liquid proportion—to instabilityof the dispersion. The liquid for moistening the coating area is thedispersion liquid or another liquid. The porous coating area ensues e.g.from the porosity of the crucible base body or from the porosity of asurface layer of the crucible base body.

The absorbent crucible base body leads to a particularly intimate bondwith the slip layer and the diffusion barrier layer to be producedtherefrom. This facilitates the application of a uniformly thick sliplayer, especially also on corners and edges and contributes to ahomogeneous heat transition on the whole crucible surface.

It has also turned out to be advantageous when the slip layer ismechanically compacted or densified. Densification can be carried outdirectly upon, application of the slip layer. Suitable methods producecompressive or shear forces in the layer, e.g. spreading or applicationwith a doctor blade. With a mechanical densification the existing gapvolume between the SiO₂ particles is reduced in size and the dispersionliquid contained therein is pressed out, thereby collecting on the freesurface. In this process it can entrain SiO₂ nanoparticles and transportthem to the surface of the slip layer. A liquid film which contains SiO₂nanoparticles and which tends to form a casting skin during drying isthereby formed on the surface. Moreover, mechanical densification alsoyields a more intimate contact of the SiO₂ particles with one another,which effects an enhanced green strength of the slip layer after dryingand improved interlocking with the inner wall of the crucible. Theporosity remaining after drying of the slip layer is preferably lessthan 15%.

It has turned out to be useful when the slip layer is produced with alayer thickness of not more than 3 mm, preferably not more than 1.5 mm.

With layer thicknesses of the slip layer of more than 3 mm—just like, inthe case of high weight percentages of SiO₂ nanoparticles of more than10%, the risk of shrinkage cracks during drying and sintering rises.This can be explained by the fact that a certain penetration of thegreen layer with rather coarse SiO₂ particles that contribute to theinterlocking of the casting skin and thereby counteract breakage duringdrying or sintering is important for preventing the formation of cracks.Therefore, the preferred thickness of the slip layer is not much higherthan the D₅₀ value of the particle size distribution in the coarsefraction of the SiO₂ particles. The layer thicknesses obtained afterdrying of the slip layer into the green layer are advantageously withinthe range of 0.1 to 1.5 mm, preferably in the range of 0.5 to 1.5 mm,and particularly preferably in the range of 0.7 to 1 mm.

Preferably, the coarse fraction consists of splintery, amorphous SiO₂grains with a grain size distribution having a D₅₀ value in the rangebetween 3 μm and 30 μm.

The splintery SiO₂ grains contribute to the integrity of the slip layerand to the interlocking with the surface and improve the adhesionthereof. The effect regarding interlocking and improved adhesion is mosteasily achieved with a specific grain size distribution that has a D₅₀value between 3 μm and 30 μm. A D₅₀ value of less than 3 μm results in asignificantly increased drying shrinkage of the slip layer, and grainshaving a D₅₀ value of more than 30 μm counteract a high solid density inthe slip, which also contributes to an increased drying shrinkage. Thesplintery grains are produced in the simplest way by grinding,preferably by wet grinding.

Preferably, use is made of a dispersion that is free of binders.

The entry of impurities into the slip is avoided in the absence ofbinders. Sintering is preferably carried, out without the help ofsintering aids. The layers produced thereby are distinguished by a highpurity. SiO₂ contents of more than 99.99% are achieved. Above all alkalielements, which may occur either as a constituent or also as impuritiesin the binder additions, lead to the formation of cristobalite atelevated temperatures. Such devitrification processes can impede thedensification in the sintering process.

As for the solar crucible for use in a crystal growing process, theabove-indicated technical object, starting from a solar crucible of theaforementioned type that is provided with a grain layer, is achievedaccording to the invention on the one hand in that the SiO₂-containinglayer is formed as a grain layer that contains amorphous SiO₂ particleswhich form a coarse fraction with particle sizes in the range between 1μm and 50 μm and a fine fraction of SiO₂ nanoparticles with particlesizes of less than 100 nm, wherein the weight percentage of the SiO₂nanoparticles in the grain layer is in the range between 2 and 15% bywt.

As for the purity and porosity, the crucible base body does not have tomeet high demands. In the simplest case it is a commercially availablesolar crucible of porous transparent fused silica or porous opaque fusedsilica of average purity.

The inner wall of the solar crucible according to the invention isprovided with a SiO₂-containing grain layer that, as intended, isthermally densified into the diffusion barrier layer only in the crystalgrowing process during heating up of the silicon.

On account of the tendency of the crucible base body to crystallize andbecause of the high temperatures, as are fundamentally needed for thedensification of a grain layer having a high silicic acid content, theproduction of a dense diffusion barrier layer from SiO₂-containing grainlayer before the intended use of the crucible encounters difficulties.To significantly reduce the “red zone” in the silicon block, low ironcontents of less than 2.5 wt. ppm, preferably less than 0.5 wt. ppm, areneeded. It has been found that this requires not only a dense diffusionbarrier layer, but also that the density of the diffusion barrier layerupon heating up of the silicon charge must be achieved as rapidly aspossible, i.e. before the melting temperature is reached.

Although sintering aids and binders contribute to a rapid sintering ofthe grain layer, they promote the tendency of the layer tocrystallization at the same time and are therefore not suited.Nevertheless, in order to achieve a rapid and reliable densification ofthe diffusion barrier layer, the invention provides a very sinter-activegrain layer. The high sinter activity thereof is achieved by way of amultimodal grain size distribution in the case of which amorphous SiO₂particles with particle sizes in the range between 1 μm and 50 μm(coarse fraction) and SiO₂ nanoparticles (fine fraction) are providedwith a weight percentage between 2 and 15% by wt. (weight percentagesbased on the solids content of the grain layer).

The sintering capacity of the grain layer depends particularly on thecomposition thereof in the near-surface region. The proportion of SiO₂nanoparticles is here decisive. A high proportion yields an increasedsintering activity that permits thermal densification at a comparativelylow temperature or with a short sintering period into a glass ofincreased density and reduced porosity. Ideally, only relatively fineSiO₂ particles are found in the near-surface region of the grain layer.

The above-explained method of the invention leads to this kind ofdistribution of the SiO₂ particles within the grain layer by way ofsegregation. Within the grain layer this produces an inner regionadjoining the inner wall of the crucible, in which the coarse fractionof the SiO₂ particles is predominantly found, and an outer regionadjoining the free surface of the layer, in which the fine fraction isenriched. The fine fraction of the SiO₂ particles is formed bynanoparticles.

An inhomogeneous particle size distribution is found within the grainlayer, with the transition between lower and upper region being notcontinuous or gradual, but rather distinct.

To achieve an adequately high amount of SiO₂ nanoparticles in the outerregion of the grain layer, the amount of SiO₂ nanoparticles within thegrain layer is on the whole in the range of 2-15% by wt, but because ofshrinkage problems it is preferably less than 10% by wt.

In contrast to standard grain layers that are distinguished by a largelyhomogeneous distribution of the particles sizes over the layerthickness, the grain layer according to the invention is inhomogeneousas it shows a variation in the particle size distribution over the layerthickness, with an enrichment of SiO₂ nanoparticles in the near-surfaceouter region. On account of this enrichment it is easier to dense-sinterthe grain layer than without said enrichment. This means that densesintering requires a lower sintering temperature and/or a shortenedsintering period than in the case of a grain layer with a homogeneousparticle size distribution. With very high proportions of SiO₂nanoparticles one will observe a tendency to the formation of cracksduring drying or sintering.

A particularly homogeneous, dense and sinter-active near-surface volumeportion with a relatively high proportion of SiO₂ nanoparticlesmanifests itself within the grain layer. In the near-surface volumeportion their proportion is thus higher than the mean proportion of theSiO₂ nanoparticles in the remaining grain layer.

The near-surface volume portion shows a low porosity. The thickness ispreferably in the range of 3-15 μm, particularly preferably in the rangeof 5-10 μm. In the near-surface volume portion the fine fractionconsisting of SiO₂ nanoparticles with particle sizes of less than 100 nmaccounts for a volume proportion of more than 70%, preferably a volumeproportion of more than 80%.

It has been found that such a grain layer can be sintered reproduciblyand under moderate sintering conditions (i.e., comparatively lowsintering temperature and/or short sintering period) into a dense,translucent fused silica layer with a relatively low surface roughness.These moderate sintering conditions are regularly met in the intendeduse of the crucible during heating of the silicon charge before themaximum temperature is reached in the crystal growing process, e.g. themelting temperature of the silicon (about 1,410° C.). A thin layer witha low and closed porosity of preferably not more than 10% remains herein the area of the former near-surface volume portion. This percentagevalue approximately marks the transition between open porosity (>10%)and closed porosity (<10%). Therefore, a material with a porosity ofless than 10% is here called “dense”. It has been found that such athin, but dense surface layer already represents an efficient diffusionbarrier for iron.

The final densification of the grain layer only during the intended useof the crucible simplifies the manufacturing process and avoids anotherwise needed hot treatment process. This does not rule out that thegrain layer has already been subjected to high temperatures before, e.g.for sintering the crucible wall at a temperature below the cristobaliteformation temperature or for burning-in a surface layer of Si₃N₄, whichis typically done below 1,200° C. However, a repeated heating up of thegrain layer to high temperatures may lead to undesired interactions withadjoining layer portions, so that this is not preferred.

It has been found that in the use of a solar crucible according to theinvention for producing a silicon block in a Bridgman or VGF process,“red zone” and dark cell edge can be reduced considerably or avoided.

It has turned out to be useful when the fine fraction within anear-surface volume portion of the grain layer is enriched such thatSiO₂ nanoparticles account for a volume proportion of more than 70%,preferably a volume proportion of more than 80%.

Normally, the SiO₂ nanoparticles are present in the near-surface volumeportion of the grain layer not in isolated form, but in the form ofaggregates or agglomerates that partly or completely embed the few SiO₂particles of the coarse fraction. Since the porosity in the near-surfacevolume portion is small, the volume proportion corresponds approximatelyto the weight percentage of the SiO₂ nanoparticles.

Thus, a volume proportion of more than 70% corresponds to an enrichmentby more than 10 times in comparison with the typical proportion of about7% of SiO₂ nanoparticles in the near-surface volume portion of the grainlayer.

Advantageous developments of the solar crucible according to theinvention follow from the sub-claims. Insofar as developments of thesolar crucible indicated in the sub-claims copy the procedures indicatedin sub-claims with respect to the production method according to theinvention, reference is made for supplementary explanation to the abovestatements regarding the corresponding method claims.

On the other hand, the above-indicated technical object of the inventionis achieved with respect to the solar crucible for use in a crystalgrowing process starting from a solar crucible of the aforementionedtype provided with a grain layer, also in that the SiO₂-containing layeris configured as a diffusion barrier layer which has a porosity of lessthan 10%, a thickness in the range of 0.1-1.3 mm, and a content of ironof less than 2 wt. ppm.

In this embodiment of the solar crucible according to the invention, theSiO₂-containing layer is configured as a dense diffusion barrier layer.Density is given when the diffusion barrier layer is dense at least in anear-surface region, in the sense that the near-surface region shows noopen porosity, but at best closed porosity.

Open porosity of a material manifests itself in that it is absorbent,which can be verified by way of a dye penetration test. The absence ofopen pores is assumed in the case of a porosity of less than 10%.Therefore, a material with a porosity of less than 10% is here called“dense”. The dense diffusion barrier layer—without open porosity—reducesthe comparatively rapid surface diffusion—especially of iron—and permitsonly the comparatively slower volume diffusion.

To enable the diffusion barrier layer to be operative as such, it isenough when the dense near-surface region has a thickness of a fewmicrometers. Thus, for instance a thin, but dense surface region of thediffusion barrier, layer of less than 100 μm or even less than 80 μmalready represents an efficient diffusion barrier for iron. Thethickness of the dense sintered diffusion barrier layer on the whole ishowever much greater due to the manufacturing process and is in therange of 0.1-1.3 mm, preferably in the range of 0.4-1.3 mm, andparticularly preferably in the range of 0.6-0.9 mm.

Moreover, the diffusion barrier layer has low iron contents of less than2.5 wt. ppm, preferably of less than 1 wt. ppm, and particularlypreferably of 0.5 wt. ppm.

It has been found that upon use of the solar crucible according to theinvention for producing a silicon block in a Bridgman or VGF process,both the “red zone” in the silicon block and the “dark edge” can bereduced significantly or avoided.

As for the method for producing a silicon block in a crystal growingprocess by providing a solar crucible with a crucible base body oftransparent or opaque fused silica comprising an inner wall, of which atleast a part is covered by a SiO₂-containing grain layer, theaforementioned technical object is achieved according to the inventionin that use is made of a solar crucible in which the grain layercontains amorphous SiO₂ particles that form a coarse fraction withparticle sizes in the range between 1 μm and 50 μm and a fine fractionof SiO₂ nanoparticles with particle sizes of less than 100 nm, whereinthe weight percentage of the SiO₂ nanoparticles on the grain layer is inthe range between 2-15% by wt., and that the SiO₂-containing grain layeris thermally densified upon heating up of the silicon.

In the simplest case the crucible base body is a commercially availablesolar crucible of porous transparent fused silica or porous opaque fusedsilica of average purity. The inner wall thereof is provided completelyor partly with a SiO₂-containing grain layer in advance, i.e. before thefilling in of a silicon charge. The above-explained manufacturing methodis suited for the production of the solar crucible provided with thegrain layer. Optionally, the grain layer can additionally be coveredwith a releasing agent layer, e.g. with Si₃N₄ as the releasing agent.

After loading with the silicon charge the solar crucible is heated up inthe standard way. Depending on the crystal growing method, the filled-insilicon is completely fused, or only a silicon melt is produced above asolid seed crystal arranged on the crucible bottom. In thelast-mentioned case, the bottom region of the solar crucible is kept ata temperature below the melting temperature of silicon, e.g. at atemperature of 1375° C. The heating energy is normally coupled in viaresistance heaters which are arranged outside the solar crucible andwhich form a plurality of heating zones that can be adjusted separately.After homogenization of the melt the cooling phase begins to start anoriented crystallization.

During the heating-up phase, the grain layer is dense-sintered.Densification is ideally completed before the maximum temperature isreached. It has turned out to be useful when the grain layer has reacheda density of more than 90%, preferably at least 93%, of its theoreticaldensity before the silicon melt is formed.

The dense-sintered grain layer forms an efficient diffusion barrierlayer; it reduces the in-diffusion of impurities from the crucible wallinto the silicon melt and the evolving silicon block.

In the fusion and crystallization process the contaminated opaque fusedsilica of the crucible base body crystallizes into cristobalite, whereasthe grain layer remains vitreous. During cooling the phasetransformation of the cristobalite and the associated volume change of2.8% leads to a breakage of the base body.

The silicon block of the invention which has been produced according tothe above-explained method using a diffusion barrier layer with the helpof a Bridgman or VGF process is distinguished by low material losses. Incomparison with a standard process without diffusion barrier layer, boththe “red zone” and the “dark edge” (determined with the measurementmethods LTLD and PL specified at the outset) are significantly reduced.

PREFERRED EMBODIMENTS

The invention will now be explained in more detail with reference toembodiments and a drawing. In detail,

FIG. 1 shows an SEM image of a sintering sample of a fused silica platewith an SiO₂ barrier layer applied thereto as a slip, after a sinteringperiod of 3 h at 1,300° C.,

FIG. 2 shows an SEM image of a sintering sample of a fused silica platewith an SiO₂ barrier layer applied thereto as a slip, after a sinteringperiod of 3 h at 1,500° C.,

FIG. 3 is a photo of a green layer in a reference sample,

FIG. 4 is a schematic representation showing a fused silica crucibleprovided with a grain layer according to the invention, in a side view,

FIG. 5 is an MDP (microwave detected photoconductivity) image of thecharge carrier lifetime in a typical silicon block, cut vertically,

FIG. 6 shows a comparison of the thicknesses of lateral in-diffusionlayers determined by means of MDP measurements, in silicon blocksproduced by means of solar crucibles with and without diffusion barrierlayer,

FIG. 7 shows results of photoluminescence measurements of silicon cellsfrom the corner portion, produced with and without diffusion barrierlayer,

FIG. 8 is a diagram with concentration curves of the total ironconcentration, measured on several polished structural samples in adirection transverse to the layer, and

FIG. 9 is a diagram with results of GDMS measurements on silicon blocks.

PREPARATION OF A SIO₂SLIP—FIRST ALTERNATIVE

Amorphous transparent fused silica grains are mixed into a dispersionliquid in a drum mill lined with transparent fused silica. Thetransparent fused silica grains consist of transparent fused silica thathas been produced from naturally occurring raw material and have grainsizes in the range between 250 μm and 650 μm. This mixture is ground bymeans of grinding balls of transparent fused silica on a roller block at23 rpm for a period of 3 days such that a homogeneous slip is formed. Inthe course of the grinding process the pH is lowered to about 4 due tothe dissolving SiO₂.

The SiO₂ grain particles obtained after the grinding of the transparentfused silica grains are of a splintery type and show a particle sizedistribution that is distinguished by a D₅₀ value of about 8 μm and by aD₉₀ value of about 40 μm. SiO₂ nanoparticles with diameters of about 40nm (“fumed silica”) with a weight percentage of 10% by wt. (based on thesolids content of the dispersion) are added to the homogeneous slip.After further homogenization a binder-free SiO₂ slip is obtained. Thesolids content of the dispersion is 75% by wt.; the SiO₂ content of theamorphous SiO₂ particles is at least 99.99% by wt., and the totalcontent of metallic impurities of the transition elements is less than2.5 wt. ppm. The content of iron is below 0.5 wt. ppm.

PREPARATION OF A SIO₂ SLIP—SECOND ALTERNATIVE

Instead of the transparent fused silica grains of naturally occurringraw material, use is made of SiO₂ grains of synthetically producedtransparent fused silica that has a hydroxyl group content of about 800wt. ppm. These SiO₂ grains are commercially available in a high-purityform and in different grain sizes. The SiO₂ content of the amorphousSiO₂ particles is at least 99.99% by wt. and the total content ofmetallic impurities of the transition elements is less than 1 wt. ppm.The content of iron is below 0.1 wt. ppm.

The dispersion of deionized water and, amorphous SiO₂ grains with a meanparticle size of about 15 μm (D₅₀ value) is homogenized without grindingballs. SiO₂ nanoparticles with diameters of about 40 nm (“fumed silica”)are added to the homogeneous slip. After further homogenization abinder-free SiO₂ slip is obtained, in which the SiO₂ nanoparticles havea weight percentage of 8% by wt. (based on the solids content of thedispersion), the total solids content of the dispersion being 75% by wt.

Preliminary Test—Sample 1

The crucible material of opaque fused silica has an open porosity andforms an absorbent substrate for slip casting. On this material (body),a slip layer is produced from the binder-free slip by application with adoctor blade (also called “casting”). In this process a SiO₂ slip layerwith a thickness of about 1 mm is applied with a doctor blade onto thehorizontally supported plate, and directly thereafter a mechanicalpressure is exerted on the slip layer by means of the doctor bladedevice.

A thin liquid film forms on the densified slip layer applied in this wayand a homogeneous and closed surface layer is formed during subsequentsurface drying in air.

This creates a body structure which macroscopically leads to theformation of a dense layer of uniform thickness which as a green layerand also as a sinter layer in the sintered state forms an intimateadhesive bond with the crucible substrate. A high segregated finefraction in the upper portion of the layer (casting skin) can be seenunder the microscope. Within, the casting skin, the fraction of fineSiO₂ particles and particularly of SiO₂ nanoparticles is much higherthan in the remaining slip layer.

The manner of applying the complete layer thickness in one operationprovides, on the one hand, a sufficiently large reservoir of SiO₂nanoparticles, which is suited for segregation on the surface, and anexcessively fast drying of the layer in air is prevented on the otherhand, which otherwise would counteract segregation and formation of thecasting skin. Consequently, this leads to a slower drying of about 3-5min and a solidification of the slip layer into the supporting layerwhich permits the formation of a substantially smooth casting skin.

During casting the slip layer is given its final shape by the action ofa tool, such as a doctor blade, a brush, a spatula, or an outlet nozzlefrom which during application a continuous slip jet exits. Owing to thespreading action of the processing tool the layer surface becomesslightly more liquid, which facilitates the enrichment of SiO₂nanoparticles also at a comparatively low liquid content. This outcome,i.e., no significant reduction of the liquid content of the slip, canalso be expected from other application techniques (such as spraying on(“Aufspritzen”)), in which the slip layer is produced with its wholethickness at once and without division into fine drops of less than 1mm.

The slip layer produced in this way is dried within 3 minutes into asupporting layer and subsequently dried—still at a slow pace—by beingallowed to stand in air for 1 hour. The casting skin is here given awax-like appearance. The complete drying is carried out in air for 4 to8 hours by using an IR radiator.

The dried slip layer has a mean thickness of about 0.8 mm. It is alsocalled “green layer”. The SiO₂ nanoparticles enriched in the surfaceregion of the green layer show a high sinter activity and improve thedensification of the layer. FIG. 1 shows the layer produced in thismanner after sintering for three hours in a sintering furnace at atemperature of around 1,300° C. Complete densification is not yetachieved in this process. The rough rugged fracture surface with manyfinely distributed pores can clearly be seen.

By comparison, FIG. 2 shows the appearance of a similar sinter layerafter sintering for three hours at an elevated temperature of 1,500° C.A crack-free and substantially smooth, strongly densified and vitrifiedfracture surface of opaque fused silica with a density of about 2.1g/cm³ is obtained. The surface layer shows a closed porosity of lessthan 5% and a thickness of about 750 μm. (Note: The transition from opento closed porosity is in the range of 5-10% according to theliterature).

Neither the layer of FIG. 1 nor the layer of FIG. 2 hints at possiblecrystallization products, neither in the inside nor on the surface.

The high sinter activity of the grain layer produced according to theinvention is apparent from the details given in Table 1. Here, sinterduration and sinter temperature and the respective sinter result aresummarized in a crosstab.

TABLE 1 0.5 h 1 h 3 h 5 h 1,300° C. opaque/porous opaque/porous opaque/opaque/ porous porous 1,350° C. opaque/porous opaque/porous opaque/opaque/ porous porous 1,375° C. opaque/porous opaque/porous translucent/translucent/ dense dense 1,400° C. translucent/ translucent/translucent/ dense dense dense 1,450° C. translucent/ dense

“Opaque/porous” means that the necessary density of the layer is missingand that this layer is not suited as a diffusion barrier layer withinthe meaning of the invention. The porosity is more than 10% and thedensity is less than 90% of the theoretical density of transparent fusedsilica (about 2.2 g/cm³).

“Translucent/dense” means that the densified sinter layer has a densityof at least 90%, preferably at least 95%, of the theoretical density, sothat it is suited as a diffusion barrier layer within the meaning of theinvention.

In crystal growing processes the crucible wall is heated up totemperatures in the range between 1,375° C. and about 1,450° C. On theassumption that when a silicon charge is heated up in a crucible, atleast one hour will typically pass until the maximum temperature isreached, the grain layer normally has the status “translucent/dense” andhas thus reached a density of at least 95% of its theoretical density.This is only true to some extent for the maximum temperature of 1,375°C. which the crucible bottom has in the so-called “quasi mono process”.In this case the desired density of the grain layer will only be reachedafter 3 hours.

Reference Example—Sample 2

The above-described, binder-free SiO₂ slip has a low viscosity and canbe used as such directly as a spray slip. In a test this slip was usedfor producing a spray coating on the absorbent, opaque fused silica bodywith open porosity.

For coating purposes the opaque fused silica plate was introduced inhorizontal orientation into a spay chamber and the top side wassuccessively provided by spraying of the slip with a supporting SiO₂slip layer having a thickness of about 0.7 mm. A spray gun which iscontinuously supplied with the spray slip was used for this purpose.

A rough and rugged surface layer is formed on the successively appliedslip layer during subsequent surface drying in air within one minute.This result is at any rate partly due to the fact that the drying of theslip layer took place so rapidly because of the porous substrate that asegregation of the fine fraction in the upper region of the slip layerwas not possible, so that a dense and closed casting skin could notform.

The further drying process then took place at a slow pace in that theslip layer was allowed to stand in air for eight hours. Complete dryingis carried out in air for 4 hours by using an IR radiator.

This yields a rough and cracked inhomogeneous surface layer of opaque,porous fused silica which has the appearance shown in FIG. 3.

The dried green layer could subsequently be sintered in a sinteringfurnace at a temperature of about 1,410° C. into a densified sinterlayer of translucent fused silica with a density of about 2.0 g/cm³.This density is still acceptable for a diffusion barrier layer.

Coating of a Crucible

The coating of a commercially available opaque fused silica crucible 1,produced from transparent fused silica of naturally occurring rawmaterial, took place in a multistage setup, which is schematically shownin FIG. 4.

The SiO₂ grain layer 2 to be formed for the diffusion barrier is appliedon all sides (bottom and sidewalls) to a pre-moistened inner wall of thecrucible 1 of porous opaque fused silica which is temperature-controlledto room temperature. Layers of different thickness and geometricaldesign can thereby be produced. In the embodiment the grain layer wasproduced by using the above-described SiO₂ slip layer—firstalternative—and on the basis of the method described in the preliminarytest—sample 1—for application (by doctor blade) and drying of the sliplayer. The green layer (=grain layer 2) produced in this way shows alargely uniform average thickness of about 0.8 mm after drying. Thethickness of the diffusion barrier layer obtained therefrom aftersintering (in the crystal growing process) is about 10% smaller and isthus about 0.7 mm. After application of said grain layer 2 the layer wascoated with a suspension of silicon nitride, silica and DI water (layer3).

The coated opaque fused silica crucible was used in a crystal growingprocess which shall be explained in more detail hereinafter. The grainlayer was here sintered and thus thermally densified into a densediffusion barrier layer with a mean thickness of about 0.6 mm.

Production of a Silicon Block

Crystal growing for producing a silicon block was carried out by usingthe coated opaque fused silica crucible and otherwise in a standardprocess as e.g. described in DE 10 2005 013 410 B4.

During heating in the crystal growing process a densified SiO₂ barrierlayer with an average thickness of about 0.6 mm and a porosity of lessthan 10% evolves from the grain layer. This barrier layer shows no openporosity and is efficiently active as a diffusion barrier layer withrespect to the fast surface diffusion of impurities, especially of iron.

Measurement Results

FIG. 5 shows a typical cross-section of the silicon block, measured bymeans of “MDP” (“microwave detected photoconductivity”). With thismeasurement method the minority charge carrier lifetime (LTLD) isdetermined in a semiconductor material. The minority charge carrierlifetime is normally illustrated in a false-color representation—in FIG.5 the illustration is kept in gray scales and the gray values correspondto the actually colored regions supplemented with the details “g”(green), “b” (blue) and “r” (red). A long charge carrier lifetime isassigned to the green and blue measurement points. The false-colorrepresentation is scaled such that the red color is assigned to thezones with a short charge carrier lifetime (LTLD<6 μs). Thus, the zonescolored in red represent the in-diffusion zones with short chargecarrier lifetime. They represent the inferior material region, which ishere called “red zone”. The cake thickness to be removed depends on thethickness of the red zone.

The “red zone” can clearly be seen on the bottom and in the lateralblock regions; it is here predominantly caused by solid-state diffusionof impurities from the solar crucible.

The diagram of FIG. 6 shows a comparison of the in-diffusion zones madevisible by way of MDP measurements for a silicon block from a cruciblewithout diffusion barrier layer (measurement series (a)) and for asilicon block from a crucible with diffusion barrier layer (measurementseries (b)). On the ordinate, width d of the lateral in-diffusion zonewhich is measured at several height positions of the silicon block ishere respectively plotted (in mm). The measurement points of about thesame height position of both measurement series are respectivelyrepresented as triangles, circles and squares.

It is apparent therefrom that in the silicon block from the cruciblewith diffusion barrier layer (measurement series b) both the lateralin-diffusion zone on the whole and the area of short charge carrierlifetime (red zone) are comparatively thin.

FIG. 7 shows representations of processed corner wafers in which thecell efficiency is made visible by way of photoluminescence. The twocorner wafers are taken from silicon blocks of the same block height.The wafer shown in figure (a) derives from a silicon block which wasproduced in a crucible without diffusion barrier. The corner wafer showstwo dark edge regions that have a lower charge carrier concentration dueto harmful in-diffusion. The corner wafer according to figure (b)derives from a block with diffusion barrier. In this wafer, the darkedge region is much smaller.

In the iron concentration profiles of FIG. 8, the concentration of ironC_(Fe) is plotted in wt. ppm against the measurement position P (in μm).The profiles were measured on polished cross sections of the coatedcrucible after completion of a crystal growing process.

The total concentration of the iron contamination was determined byinductively coupled plasma mass spectrometry (ICP-MS). The samplesdesignated with “SL1” and “SL2” were prepared by means of crucibles withslip-based diffusion barrier layer according to the invention. In thecomparative sample designated with “PLA”, the diffusion barrier layerwas produced by using insertion plates of transparent fused silica, asdescribed in the above-mentioned DE 10 2011 082 628 A1. The value zeroon the x-axis marks the boundary between the crucible wall (substrate S)and the respective diffusion barrier layers (L).

It can be seen that the diffusion barrier layers L after the Si meltingprocess show much lower Fe concentrations than the crucible material(substrate). A diffusion profile is perceivable. The higherconcentration level of the slip-based layers (SL1 and SL2), whichoriginally during application were much purer (Fe level about 0.2 wt.ppm, i.e. similar to the concentration level of the plate sample), isnoteworthy. This is an indication that a certain enrichment with Fe thatpenetrates through the whole layer already occurs during the heating upor densification, respectively, of the sample. This leads to anessential technical demand for a densification of the layer that is asfast as possible.

It is noteworthy that the SiO₂ initial slip has a Fe initialconcentration that corresponds approximately to the asymptotic level ofthe SiO₂ plate material (“PLA”). The fact that the Fe concentrationlevel after the melting process has risen slightly above 1 ppm isindicative of an enrichment of the whole layer during the densificationprocess. Since it must be presumed that the layer which is stronglydensified at the end probably shows a similar behavior as the platematerial that is dense right from the beginning, this is in support ofthe assumption that the initial porosity promotes this enrichment, andof the conclusion that the material should densify at temperatures thatare as low as possible (below 1410° C.).

FIG. 9 shows results of a GDMS measurement (glow discharge massspectrometry) for a silicon reference block (called “Ref”) producedwithout a diffusion barrier layer and a silicon block (called “SL”)produced with a diffusion barrier layer according to the invention, byway of comparison. On the ordinate of the diagram, the respective metalcontent (iron, copper) C_(Me) is here plotted in wt. ppm on alogarithmic scale against the distance A (in mm) from the crucible wall.It is apparent therefrom that the metal concentration is decreasing withan increasing distance and that a reduction of the iron and coppercontent in the finished silicon block could be achieved by using theSiO₂ diffusion barrier. It is striking that in the sample according tothe invention with a diffusion barrier layer for iron (SL-Fe) it is notonly the edge concentration but also the whole profile that is clearlylower than in the reference sample (Ref-Fe) without diffusion barrierlayer.

The invention claimed is:
 1. A method for producing a solar cruciblewith a rectangular shape for use in a crystal growing process forsilicon, the method comprising: providing a crucible base body oftransparent or opaque fused silica comprising an inner wall; providing adispersion containing amorphous SiO₂ particles; applying aSiO₂-containing slip layer with a layer thickness of at least 0.1 mm toat least a part of the inner wall by using the dispersion; drying theslip layer so as to form a SiO₂-containing grain layer having a castingskin; and thermally densifying the SiO₂-containing grain layer so as toform a diffusion barrier layer, wherein the dispersion contains adispersion liquid and the amorphous SiO₂ particles that form a coarsefraction with particle sizes in the range between 1 μm and 50 μm and afine fraction with SiO₂ nanoparticles with particle sizes of less than100 nm, the fine fraction with the SiO₂ nanoparticles with particlesizes of less than 100 nm accounting for a volume proportion of thecasting skin of more than 70%, wherein a weight percentage of the SiO₂nanoparticles based on a solids content of the dispersion is in therange between 2 and 15% by wt., and wherein the SiO₂-containing grainlayer is thermally densified into the diffusion barrier layer throughheating up of the silicon in the crystal growing process.
 2. The methodaccording to claim 1, wherein the solids content of the dispersion isless than 80% by wt.
 3. The method according to claim 1, wherein thedispersion is free of binders, wherein the SiO₂ content of the amorphousSiO₂ particles is at least 99.99% by wt., and wherein a total content ofmetallic impurities of transition elements is less than 5 wt. ppm. 4.The method according to claim 1, wherein the slip layer is applied bycasting the dispersion onto the inner wall.
 5. The method according toclaim 1, wherein the inner wall is moistened prior to the application ofthe slip layer, and wherein the inner wall of the crucible base body isa porous inner wall.
 6. The method according to claim 1, wherein a greenlayer obtained after drying of the slip layer has a layer thickness inthe range of 0.1-1.5 mm.
 7. A method for producing a silicon block in acrystal growing process comprising: providing a solar crucible with acrucible base body of transparent or opaque fused silica comprising aninner wall, of which at least a part is covered by a SiO₂-containinggrain layer having a casting skin; and filling the solar crucible withsilicon, the silicon being heated so as to form a silicon melt, thesilicon melt being cooled down with crystallization and formation of thesilicon block, wherein the SiO₂-containing grain layer containsamorphous SiO₂ particles that form a coarse fraction with particle sizesin the range between 1 μm and 50 μm and a fine fraction of SiO₂nanoparticles with particle sizes of less than 100 nm, the fine fractionof SiO₂ nanoparticles with particle sizes of less than 100 nm accountingfor a volume proportion of the casting skin of more than 70%, whereinthe weight percentage of the SiO₂ nanoparticles of the SiO₂-containinggrain layer is in the range between 2 and 15% by wt., and wherein theSiO₂-containing grain layer is thermally densified during heating up ofthe silicon.
 8. The method according to claim 7, wherein before theformation of the silicon melt, the SiO₂-containing grain layer hasreached a density of more than 90% of its theoretical density.